When magnetizing the measured ferromagnetic material with a magnetizer, if the material is continuous and uniform, the magnetic induction lines in the material will be constrained in the material. The magnetic flux is parallel to the surface of the material, and there is almost no magnetic induction line coming out from the surface and no magnetic field on the surface of the inspected workpiece.
However, when there is a defect in the material that cuts the lines of magnetic force, the defect on the surface of the material or the change in the organizational state will change the permeability. Due to the small permeability of the defect and large magnetic resistance, the magnetic flux in the magnetic circuit is distorted, and the flow direction of the magnetic induction line will change. In addition to part of the magnetic flux directly passing through the defect or through the material to bypass the defect, some of the magnetic flux will leak to the surface of the material, bypassing the defect and re-entering the material through air. Thus, a leakage magnetic field is formed at the surface defect of the material. We can detect the distribution and size of the leakage magnetic field with the magnetic sensitive sensor. And in this way, to achieve the purpose of magnetic flux leakage ndt.
In magnetic non-destructive testing, magnetization is the first step to achieve magnetic flux leakage testing, which determines whether a leakage magnetic field signal can be generated, and it also affects the performance characteristics of the detection signal and the structural characteristics of the detection device. Like the magnetic particle inspection, the magnetization is realized by the magnetizer, which includes the magnetic source and the magnetic circuit. Depending on the structure of the tested component, both the magnetic source and the magnetic circuit will change.
Magnetization methods can generally be divided into five categories, including AC magnetization method, DC magnetization method, permanent magnetization method, composite magnetization method, and comprehensive magnetization method.
In the process of magnetic flux leakage testing, it is usually required that the magnetic induction in the ferromagnetic member be above 0.7 Tesla, or the coil magnetization ability is calculated at 5A/mm2.
In magnetic detection, the volume and weight of the detection device are mainly concentrated on the magnetizer, which in turn determines the field performance of the detection device. Therefore, the intensity should be selected to reduce the weight of the magnetizer as the main goal while ensuring detection sensitivity.
(1) Only applicable to ferromagnetic materials.
(2) The detection sensitivity is low.
(3) The quantification of defects is rough.
(4) Due to the limitation of the shape of the workpiece to be detected, the method of using sensors to detect magnetic flux leakage and magnetic flux leakage is not suitable for detecting specimens with complex shapes.
(5) Magnetic flux leakage testing is not suitable for very narrow cracks, especially closed cracks. The experiment found that the fatigue cracks with very narrow cracks, magnetic particle testing, and magnetic flux leakage testing all failed to produce damage display and signal.
(1) Application of magnetic flux leakage ndt in the iron and steel industry: In steel plants, it is mainly used to inspect steel structural parts, billets, round steel, bars, steel pipes, welds, and steel cables to confirm the integrity of the finished product. In many cases, users will not accept steel products that have not been inspected by steel mills and third parties. Users often use magnetic flux leakage testing before manufacturing, which can ensure the manufacturer's technical requirements for the product. Such inspections are often carried out by independent testing companies or the user's quality assurance department.
(2) Application of magnetic flux leakage ndt in the petrochemical industry: Test installed oil and gas pipelines (including buried pipelines), oil storage tank bottom plates, or recovered oilfield steel pipes.
(3) Perform regular in-service flaw detection on used steel cables, wire ropes, and chains.